1. Sensitivity Studies Using Nested HYCOM Models 2004 Layered Ocean Model Users’ Workshop February 9-11, 2004 RSMAS, Miami, FL Patrick Hogan Luis Zamudio Alan Wallcraft E.Joseph Metzger Naval Research Laboratory Stennis Space Center, MS

2. Encl (7) Purpose Provide accurate, generalized nesting capability to support littoral applications for the next Navy global prediction system (based on HYCOM, 2006 transition). Evaluate the next generation coastal ocean models (HYCOM, NCOM, perhaps others (ROMS?)). Develop coastal modeling capabilities for HYCOM. Model-model and model data inter-comparisons Approach Determine the sensitivity to nesting techniques and parameters, the impact of nesting with like and unlike model design and/or vertical distribution. Examine the impact of nesting on ocean dynamics with emphasis on the accurate propagation of oceanic features from different dynamical regimes across nested model boundaries. Implement and evaluate coastal/littoral HYCOM HYCOM Coastal Nesting Strategy

3. Provides a benchmark for model-model comparisons Evaluate sensitivity to vertical remapping methods Evaluate methods for lateral transmission of information Accuracy of representation of coastal dynamical processes Propagation of deep water events into the coastal region First, nest coastal HYCOM and NCOM within basin- scale HYCOM at the same horizontal resolution Initial approach: Some preliminary results Gulf of California HYCOM to HYCOM, same vertical structure HYCOM to NCOM-like, 26-layer Hybrid to 40-level sigma-z Gulf of Mexico

4. Sensitivity Studies for HYCOM to HYCOM Nesting Relaxation (nudging) within a sponge zone for the baroclinic mode The Gulf of California a good test bed for coastally trapped waves Can be generated by Hurricane or El Niño Larger GoC used to examine impact of waves generated inside domain (Metzger) Smaller GoC will be used to examine impact of boundary condition on externally generated waves Method of Characteristics for the barotropic mode Current status of nesting in HYCOM “off-line”, boundary information comes from archive files Don’t need to know nested area in advance, but updating frequency limited by archive file frequency

5. Gulf of California Nesting Sensitivity Studies 8 km Pacific HYCOM provides boundary Conditions to Gulf of California HYCOM Hurricane generated within domain Hurricane signal must pass through boundary 24th 27th (all models have 8 km resolution) The updating frequency (both modes) The e-folding time across the buffer zone The width of the buffer zone Exclude the baroclinic/barotropic mode Which variables are updated

6. The smaller Gulf of California domain must accurately pass the coastal- trapped waves and eddies across the nested boundary Historical HYCOM nesting parameters: Method of characteristics for Barotropic mode (u,v,p) 1-day updating frequency Relaxation to outer grid solution in Buffer zone for baroclinic mode 10 gridpoint wide relaxation zone 1-10 day relaxation e-folding time 6-day updating frequency Open boundaries 20-layers Non-assimilative Forced with NOGAPS

7. Standard Nesting Parameters RMS error map (wrt Pacific model over GoC domain) Time series of domain- wide RMS error Different variables respond differently Same geometry Same horz. res. Same vert. res.

8. Sensitivity to: Updating frequency 3 hours, 1,2,4,6 days 10 gridpoints 0.1-1.0 e-folding Barotropic + baroclinic E-folding time in BZ 1-10, 1-5,.1-1,.1-.5,.1-.2 10 gridpoints 3 hours Barotropic + baroclinic

9. Sensitivity to: Barotropic/baroclinic mode Width of buffer zone Barotropic or baroclinic only 10 gridpoints 0.1-1.0 e-folding 3 hourly 1,2,3,4,5,10 gridpoints 3 hourly 0.1-1.0 e-folding Barotropic + baroclinic

10. Lowest Error Nesting Parameters Time series of domain-wide RMS error RMS error map Hurricane Jullitte passes through boundary Hurricane Julliette forms (outside of boundary) 1-10 gridpoints.1-1 day e-folding 3 hour updating Baroclinic+barotropic * Matzalan

11. SSH Anomaly vs. Matzalan Tide Guage Station E-folding timeUpdating frequency Width of buffer zoneBarotropic/Baroclinic

12. 1/12° Pacific HYCOM Basin-scale Circulation with nested US West Coast HYCOM Forced with high frequency ECMWF winds and thermal forcing SSH Snapshot – 21 March

13. 1/12° Pacific HYCOM Basin-scale Circulation with nested US West Coast HYCOM Pacific Basin HYCOMNested US West Coast HYCOM Nested HYCOM uses same resolution and forcing as Pacific HYCOM to test bc. Simulation begins on January 1, 2001; no data assimilation is included.

14. ~8 km (1/12°) Atlantic HYCOM IAS nest Portugal nest Florida Current nest Mississippi Bight nest Non-assimilative; forced with NOGAPS Gulf of Mexico

15. Nested Gulf of Mexico 8 km to 8 km SSH snapshot from ATL model used to restart and and provide boundary conditions to GoM model ATL model has 20 m coastline Nested Gulf of Mexico SSH 1-day after restart from ATL GoM has 5 m coastline with 10 m minimum depth 10 gridpoint buffer zone 1-10 day e-folding 1 day updating for Barotropic and baroclinic

16. Snapshot of SSH from 8 km ATL model Snapshot of SSH from 8 km Gulf of Mexico model 4 months later Boundary conditions Coastline change Nested Gulf of Mexico 8 km to 8 km

17. 8 km 40-level sigma-z GoM model restarted from 26 layer hybrid GoM model (1 month of adjustment to new coastline allowed for) 1 day after restart Vertical remapping via PLM Smooth adjustment to different vertical descritization 8 km 26-layer hybrid GoM model 1 month after restart from 8 km ATL model 26-layer hybrid vs. 40-level sigma-z (“HYCOM in NCOM mode”) True NCOM – full cell topography HYCOM/NCOM –partial cells

18. 8 km 40-level sigma-z GoM Both 3 months after restart from “adjusted” nested GoM model Similar solutions after 3 months despite different vertical discritization 8 km 26-layer Hybrid GoM

19. Zonal cross-section (26.9N) of Salinity on December 1, 1999 (4 mo.) 1000 40-level sigma-z 26-layer hybrid Vertical coordinate overlaid Salinity contours overlaid Shelf-break front

20. Determine the optimal vertical configuration (?) Several test cases: Different dynamical regimes HYCOM to HYCOM (different horizontal/vertical resolution, etc.) NCOM to NCOM, ROMS(?) (coupling different model designs) HYCOM to HYCOM with fixed vertical coordinates over the shelf HYCOM to NCOM (using HYCOM) with full/partial-cell topography In general, Anything to Anything What is the impact of possibly retaining isopycnals over the shelf? What is the impact of full-cell topography in NCOM’s z-level mode? What is the impact due to the location of the open boundary? Near Future Plans Add coastal capabilities to HYCOM Realistic River plume dynamics Tidal forcing Realistic bottom boundary layer Treatment of the free surface

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